FIG. 21 is a diagram illustrating a conventional direct conversion circuit used in a matrix converter or the like of a direct conversion scheme (also referred to as AC direct scheme) cited in a number of documents such as Patent Literature (PTL) 1.
With the conventional direct conversion circuit illustrated in FIG. 21, AC switches S1 to S9 each of which has a breakdown voltage against both positive and negative voltage polarities are disposed as illustrated in FIG. 21.
As illustrated in (a) in FIG. 21, each of the AC switches (S1 to S9) has a configuration having two circuits connected in series, in which diodes and insulated gate bipolar transistors (IGBT) are connected in parallel to be oriented oppositely. Alternatively, as illustrated in (b) in FIG. 21, each of the AC switches has a configuration in which two IGBTs having reverse breakdown voltage are connected in parallel to be oriented oppositely.
The IGBT has a feature of having a high breakdown voltage with smaller conduction loss among high-current operation devices. Accordingly, in a matrix converter of the conventional directly conversion scheme (AC direct scheme), the IGBT is used as a switching element used in the AC switches. However, since the IGBT can pass through a current only unilaterally and the breakdown voltage in an opposite direction is weak, the configuration having two circuits connected in series, in which diodes and IGBTs are connected in parallel to be oriented oppositely as illustrated in (a) in FIG. 21 is generally employed. In addition, an IGBT whose reverse breakdown voltage is increased is also developed. With this, a reverse blocking IGBT in which IGBT chips are connected inversely in parallel to face each other is also used as an AC switch, as illustrated in (b) in FIG. 21.
Meanwhile, a power switching element formed of a nitride semiconductor which differs from a power switching element of silicon series such as IGBT is under development. Techniques related to an inverter using a feature of the power switching element is disclosed by PTL 2 and so on.